Background Several environmental exposures, including infection with Epstein-Barr virus, low levels of vitamin D and smoking are established risk factors for multiple sclerosis (MS). Also, high hygienic standard and infection with parasites have been proposed to influence MS risk. The aim of this study was to investigate the influence of various environmental exposures on MS risk in a Norwegian cohort, focusing on factors during childhood related to the hygiene hypothesis.Methods A questionnaire concerning environmental exposures, lifestyle, demographics and comorbidity was administrated to 756 Norwegian MS patients and 1090 healthy controls. Logistic regression was used to calculate odds ratio (OR) with 95% confidence interval (CI) for the risk of MS associated with the variables infectious mononucleosis, severe infection during childhood, vaccination and animals in the household during childhood. Age, gender, HLA-DRB1*15:01, smoking and infectious mononucleosis were included as covariates. General environmental exposures, including tobacco use, were also evaluated.Results. Infectious mononucleosis was confirmed to be significantly associated with increased MS risk, also after adjusting for the covariates (OR=1.79, 95% CI: 1.12-2.87, p=0.016). The controls more often reported growing up with a cat and/or a dog in the household, and this was significant for ownership of cat also after adjusting for the covariates (OR=0.56, 95% CI: 0.40-0.78, p=0.001). More patients than controls reported smoking and fewer patients reported snuff use.Conclusions In this Norwegian MS case-control study of environmental exposures, we replicate that infectious mononucleosis and smoking are associated with increased MS risk. Our data also indicate a protective effect on MS of exposure to cats during childhood, in accordance with the hypothesis that risk of autoimmune diseases like MS may increase with high hygienic standard.

The hygiene hypothesis is a hypothesis that states that a lack of early childhood exposure to infectious agents, symbiotic microorganisms (e.g. gut floraor probiotics), and parasites increases susceptibility to allergic diseases by suppressing the natural development of the immune system. In particular, the lack of exposure is thought to lead to defects in the establishment of immune tolerance.

We all know about smoking and EBV infection so what next. If you look hard enough will you find a link and this one was having cats which reduced the risk by a half, will it reproduce.So is it time to get a Moggie...maybe good for MS...However remember it could be bad for other things like allergies or your local bird life and Mice:-). I am sure many of you had cats in the house

The question is what are we going to link MS to next, high heels after all MS affects women more than men. Crazy idea?